Frequency modulation



May 5, 1942. H. TUNlCK FREQUENCY MODULATION Filed Aug. 29, 1941 4Sheets-Sheet 1 y 5, 1942- H. TUNICK FREQUENCY MODULATION Filed Aug. 29,1941 4 Sheets-Sheet 2 wwm W vmm mm @8321 5:33 a wwm HARR Y TUN/ CK y1942- H. TUNICK FREQUENCY MODULATION Filed Aug. 29, 1941 4 Sheets-Sheet3 TN m mw/o mm Q WY mm/ w R M M H E? 0 MN m6 .90 w 9 4 w m 2 1% o d lmMay 5, 1942. H. TUNlCK 2,282,103

FREQUENCY MODULATION Filed Aug. 29. 1941 4 Sheets-Sheet 4 INVENTOR HARRYTUN/ CK BY w/mm ATTORNEY Patented May 5, 1942 FREQUENCY MODULATION HarryTunick, Rye, N. Y., assignor to Radio 001- poration of America, acorporation of Dela- ApplicatiorfAugust 29, 1941, Serial No. 408,738

7 Claims.

This is a continuation in part of my copending application Serial No.310,495, filed December 22, 1939. Figures 1, 2, 2a, 2b, 3 and 4 hereinare, respectively, original Figures 10, 2, 2a., 2b, 4 and 5 of my parentapplication Serial No. 310,495. This application, furthermore, is acontinuation in part of my copendlng application Serial No. 369,800,filed December 12, 1940, since Figure 5 herein is Figure of my copendingapplication Serial No. 369,800, filed December 12, 1940.

Generally, my present invention relates to angular velocity modulationand more particularly to transmitters for producing and .transmittingfrequency modulated waves. In this respect, it has been proposed toemploy oscillation generators having tuned grid and tuned plate circuitsfor the production of frequency modulated waves by varying the tuningof, say, the tuned grid circuit in accordance with the modulation. Sincethe grid circuit tuning is varied and the plate circuit remains fixed intuning, the variable frequency currents are phase distorted, i. e'.,shifted in phase in the plate circuit which is, of course, undesirable.One object of my present invention is to provide an improved system forfrequency modulated waves which shall be substantially free of this typeof distortion, namely, phase distortion. Other objects, advantages andfeatures of my invention will appear as the more detailed descriptionthereof proceeds.

In the drawings, Figure 1 is a wiring diagram of an oscillator havingseparate tuned input and output circuits andappropriate reactance tubesystems for simultaneously varying the tuning of the input and outputcircuits to thereby produce angular velocity modulated wavessubstantially free of phase distortion; Figure 2 illustrates anothermodification of my invention in which; among other things, the plate andgrid circuits of a crystal controlled oscillator are simultaneouslyvaried to produce frequency modulation of the generated waves; Figures2a and 2b are explanatory curves illustrating signal pro-emphasis andde-emphasis at the transmitter and receiver, respectively; Figure 3illustrates a modification of Figure 2; Figure 4 illustrates a systemfor producing frequency modulation simultaneously by varying the screengrid and plate voltages of an oscillation generator; and Figure 5 is amodified form of the system shown in Figure 1.

Referring to Figure 1, tubes 26, 28 are provided with a tuned inputcircuit and a tuned output or plate circuit 34. Grid circuit 2|!comprises coil 22 shunted by variable tuning condenser 24 and,similarly, the tuned plate circuit 34 consists of a coil 36 shunted by avariable condenser 38. Oscillations are generated regeneratively by thecross-connected feedback condensers 30, 32. Output is taken from the,by-passing condensers 40, 42 as illustrated. Amplified signals,asexplained in my parent application Serial No, 310,- 495, are fedthrough transformer 6 and lead 8 to the screen grids III, I2 ofreactance tubes I4, I6. In case video signals are transmitted, radiofrequency by-pass condenser I8 should be made smaller so as not toby-pass the highest video frequencies fed through transformer 6. Ifdesired, condenser I8 may be omitted.

The reactance tubes I 4, I6 act to change the tuning of the grid circuit20, comprising coil 22 and condenser 24, an amount proportional to theamplitude of the signal fed through transformer 6 and at a rate orfrequency identical to the frequency of the signal fed throughtransformer 6. As a consequence, the push-pull connected oscillatortubes 26, 28 regeneratively cross coupled by condensers 3D, 32, producefrequency modulated high frequency oscillations in the tank or platecircuit 34 comprising inductance, coil 36 and condenser 38. Thefrequency modulated oscillations are fed through condensers 40, 42 to apower amplifier and/or limiter and/or frequency multipliers andeventually to an antenna, as explained in my parent application SerialNo. 310,495.

Automatic frequency controlling voltages may be fed through lead Ib tothe suppressor grids I44, I46 of reactance tubes I4, I6 and also, asillustrated, to the suppressor grids of reactance tubes RIB, RI 4.Similarly, the signal is fed from the secondary of transformer 6 to thescreen grids RID, RI2 of reactance tubes RI4, RIG. Over-modulationcontrol voltages derived as explained in my parent case Serial No.310,495 may be inserted through resistor 234.

Reactive voltage substantially in quadrature with that existing at theupper terminal of tuned grid circuit 20 is impressed upon the grid I56of reactance tube I4 through the action of condenser I48 and resistorI52. Similarly, a quadrature voltage with respect to the voltage at thelower terminal of circuit. 20 is impressed on grid I58 through theaction of condenser I and resistor I54. The plates of the reactancetubes I4, I6 are fed with suitable plate voltage through chokes I62, I64which are appropriately bypassed by by-passing condensers I66, I68.Condensers I10 and H2 are large coupling condensers.

As already explained, a second pair of reactance tubes R14, R18 areconnected to the plate circuit 34. The suppressor grids R144 and R148 ofthe plate circuit reactance tubes R14, R18 are connected in parallel andto the suppressor grids I44, 148 of reactance tubes 14 and 18.Similarly, screen grids R10, R12 are connected in parallel and to thescreen grids 10, 12 of the grid circuit reactance tubes 14, 18. Phasedisplaced voltages are taken from the plate circuit 34 and fed to thecontrol grids R158 and R158 by means of condensers R148, R150 andresistors R152, R154.

In view of the foregoing description, it will be observed that thesignal input not only varies the tuning of the grid or input circuit 20,but it also simultaneously changes the tuning of the circuit 34. In thisway, phase distortion of the modulated currents is avoided. That is tosay, if, for example, circuit 34 were not simultaneously tuned to thegrid circuit 20, then as the circuit 20 is altered in frequency, thecircuit 34 would present undesired reactance to the changed frequency ofoscillation and, hence, would introduce phase shifts which areundesired. By simultaneously varying both the grid and plate circuitsthis defeet is circumvented.

In the transmitter of Figure 2, microphone 300 feeds an audio frequencyamplifier 302 whose characteristic is as shown in Figure 2a. That is,amplifier 302 should have a fiat characteristic along the range A to Bof about 10 to 500 or 10 to 1000 cycles. From points B to C thecharacteristic should rise, preferably linearly. B to represents theremainder of the audio frequency.

range which may be as desired from 500 or 1000 to 5000, 7500 or 10,000or more cycles. When such an amplifier is used at the transmitter, thefrequency modulation audio frequency amplifier at the receiver shouldhave a complementary characteristic, namely, fiat along the frequencyrange from A to B and then drop off from the frequency range B to C, asshownin Figure 2B.

The output of audio frequency amplifier 302 is fed through transformer304 and additional transformers 308, 308 to the grids of reactance tubes310, 312. Audio frequency by-passing condensers 314, 310 are provided asare also radio frequency chokes 318, 320. Depending upon the adjustmentsof reactance tubes 310, 312 the crystal 322 is effectively shunted byvariable capacity or inductance represented by tube 310 and the platecircuit of the crystal controlled oscillation generator tube 324 is alsoshunted by a variable inductor or capacity represented by reactance tube312. Both tubes 310, 312 should be adjusted so as to add similarreactances, namely, capacitive or inductive, to the grid and platecircuits of crystal controlled generator 324. If desired, of course, thecrystal may be replaced by a tuned circuit or a quarter wave resonantline. The audio frequency voltages applied to the reactance tubes 310,312 may be adjusted by virtue of taps 330, 332 on the potentiometershunting the secondary of transformer 304. The output of the oscillationgenerator may be fed through a limiter and/or power amplifier and/orfrequency multiplier 334 whose output is radiated by the antenna 338.

Over-modulation pickup antennae or coils 338, 340 are provided, antenna338 feeding into a circuit 342 tuned to or slightly beyond one extremefrequency of the channel assigned to the transmitter of Figure 2.Similarly, tuned circuit 342 is tuned to or slightly beyond the otherextreme frequency of the channel assigned to the transmitter. Circuit340 energizes amplifier 344, in

turn connected to rectifier 348. when the swing exceeds the channelallotted to the transmitter, an indication is produced in indicator 343and to automatically prevent this condition from persisting, anautomatic volume control lead 358 is provided. Lead 350 may be connectedto one of the audio stages of amplifier, 302 to cut down its gain whenoutput is produced in rectifier 348.

Similarly, when the transmitter goes beyond its extreme allottedfrequency in theother direction, rectifier 352 becomes active to producean indication in the bell or lamp indicator 354 and also inject a gainreducing voltage in the automatic volume controlling lead 358 leading tothe same or another amplifier stage in audio frequency amplifier 302. Ifdesired, the system may be operated without the automatic volumecontrolling feature, in which case switch 380 and/or switch 382 may beopened In the transmitter of Figure 3. reactance tubes 800, 002 are mademore conductive by the action of transformer 804 associated with thecathodes of reactance tubes 800, 802. Transformer 804 is energized bysignal amplifier 808 in turn connected, if desired, to a microphone 010.Condenser 812 and resistor 814 are adjusted so as to energize the grid818 with a relatively leading voltage. Condenser 818, coil 818 andresistance 820 are adjusted so as to impress a relatively laggingvoltage with respect to the voltage at point 822 upon the grid 824 oftube 802. The cathodes of the reactance tubes are connected to -a sourceof potential 830 such that both tubes draw a mean current in the absenceof signal from amplifier 808. The apparatus within box 840 is identicalto that within the dotted recbe adjusted so that they both draw leadingcur- In the transmitter of Figure 4, the oscillator] tube 000 may haveeither crystal 002 connected between the plate 004 and, grid 000 or, asshown, the series tuned circuit 008 is connected by means of switch 010between the plate 004 and grid 008. The screen grid 020 is supplied withvoltage through radio frequency choke 022 and by-pass condenser 924. Theplate 004 is supplied with plus voltage through choke 830 bypassed byby-passing condenser 032. Transformers 034 and 0 38 insert alternatingsignal voltages in the plate and screen voltage leads. These signalvoltages are derived from potentiometer 040 connected to the secondaryof transformer 042 whose primary is connected across the plates ofpush-pull amplifier 844 energized from audio or signaling amplifier 848.Since like voltage changes applied to the screen 020 and plate 004produce opposite efiects on the frequency of oscillation of tube 000transformers 038 and 034 are so polarized as to oppositely swing thescreen 020 and plate 004 in voltage in accordance with signal voltagesderived from amplifier 048. The output circuit 050 of the oscillator 000is fed to the limiter 052 which may be a power amplifier and/orfrequency multiplier and in turn 952 energizes the antenna 954.

In the transmitter of Figure 5, tubes 2, 4 are provided with tunablehigh frequency input and output circuits 6 and B respectively. Feed-backcausing high frequency oscillation generation is accomplished by meansof feed-back condensers Ill, l2.

Tubes l4 and I6 are connected symmetrically and to opposite sides of thetuned circuit 8 by means of condensers l8 and 20. It will beappreciated, therefore, as the tubes I4 and I6, which have their gridsconnected in parallel by means of lead 22', are made more conductive,more and more capacity is eifectively connected in shunt to circuit 8',thereby lowering the frequency of oscillation of the oscillationgenerating tubes 2', 4'.

Tubes 30, 32, which have their grids connected in parallel by lead 34,are connected to opposite sides of the plate tuned circuit 8' throughinductors 36, 38'. Hence, as the tubes 30, 32' are made more and moreconductive, more and more inductance is effectively connected in shuntwith the coil of the tuned circuit 8 and, hence, this acts to reduce thetotal inductance in circuit and, hence, raise the frequency ofoscillation.

For frequency modulating the push-pull oscillation generator 2, 4' theleads 22', 34' are connected as shown to opposite sides of the secondaryof transformer 40. The primary of the latter is fed with amplified voiceor tone currents from amplifier 42 in turn supplied by microphone 44..By making the amplifier 42 have a rising characteristic for themodulating frequencies fed to its input side, the resulting output ofthe system taken from leads 46, 48 will be phase modulated.

The tunable grid circuit of the transmitter of Figure may be similarlyvaried in frequency by means of tubes 50', 52' connected to oppositesides of the tuned circuit 6 through condensers 54' and 56 and by meansof tubes 80,

secondary of transformer 80'. The primary of source of modulating waves,and means for simultaneously varying the resonance of said input circuitand said output circuit in response to waves from said source to therebyvary the frequency of oscillations generated.

2. In combination, a push-pull connected vacuum tube oscillationgenerator having a parallel tuned plate circuit and a parallel tunedgrid circuit, a pair of reactance tubes symmetrically connected to thegrid circuit, another pair of reactance tubes symmetrically connected tosaid plate circuit, andmeans for simultaneously varying theconductivities of all of said reactance tubes in order to therebysimultaneously vary the frequency of oscillations generated by saidgenerator.

3. Apparatus as claimed in the preceding claim, characterized by theprovision of means connected to the tuned grid circuit for supplying thegrids of the reactance tubes symmetrically connected to the grid circuitwith phase displaced voltages relative to the voltages across the gridcircuit, and being further characterized by the fact that means areprovided for supplying the grids of the reactance tubes connected to theplate circuit with phase displaced voltages displaced in phase withrespect to and derived from voltages across said plate circuit.

4. In combination, an oscillation generatorhaving a tuned input circuitand a tuned output circuit, a pair of reactance tubes symmetricallyconnected to said tuned input circuit, a second pair of reactance tubessymmetrically connected to said tuned output circuit, and means forsimultaneously varying the conductivity of all of said reactance tubesand thereby varying the frequency of oscillations generated by saidgenerator.

5. In combination, a vacuum tube oscillation generator having a resonantgrid circuit and having a separate resonant plate circuit, an electrondischarge device connected to said grid circuit through a reactance, asecond electron discharge device connected to said plate circuit throughanother reactance, and means for simultaneously varying the conductivityof said tubes with control voltages to simultaneously change the tuningof said grid and plate circuits.

6. In combination, a push-pull vacuum tube oscillation generator havinga resonant grid circuit and a resonant plate circuit, a pair of electrondischarge devices connected to said grid circuit through reactors,another pair of electron discharge devices connected to said tuned platecircuit through another [pair of reactors, and means for varying theconductivities of said pairs of devices with control voltages so as tosimul taneously vary the tunings of said grid and plate circuits.

'7. In combination, a high frequency circuit, a pair of electrondischarge devices connected to said circuit through capacitivereactances, a second pair of electron discharge devices connected tosaid circuit through inductive reactances, and a circuit for oppositelyvaryin the conductivities of said pairs of devices to thereby alter thetunin! of said first-mentioned circuit.

HARRY TUNICK.

